Viscosity renormalization in colloidal suspensions

Dynamic viscosity of colloidal silica suspensions at low and high volume fractions

A comprehensive study was carried out on the dynamic viscosity of X30 silica dispersions at both high and low volume fractions of colloidal silica particles at various electrolyte ionic strength and pH values. Booth and Ruiz-Reina and Carrique theoretical models (R-R&C) were compared in predicting the primary electroviscous effect (PEE) for viscosity at low volume fractions. To this respect the colloidal dispersion was well characterised with regards to electrolyte properties such as the Debye length, κ^-1, calculated from the ionic strength, and zeta potential, ζ, calculated from the electrophoretic mobility using the full numerical model by O'Brien and White (O'B&W). R-R&C hard sphere model (which is a modified version of Simha hard sphere model that includes a boundary condition by Happel on the outer radius of the cell) and the semi-empirical Krieger-Dougherty (K-D) models were fitted to the experimental data at high volume fractions. At both low and high volume fractions the viscosity increased with pH and decreased with ionic strength. At low volume fractions both theoretical models significantly underestimated the experimental dynamic viscosities obtained in this work. This could be attributed to the fuzzy structures for silica particles in aqueous conditions reported previously in the literature, where a significantly larger electroviscous parameter, p, was obtained experimentally for silica particles. The experimental electroviscous parameter, p_exp, in this work was found to be roughly an order of magnitude up to 26 times larger than that predicted by Booth and around 5 ± 1 times larger than the predicted p by R-R&C model allowing the introduction of a correction factor in the PEE coefficient obtained from R-R&C model enabling good prediction for X30 silica dispersions by the latter model. The significant improvement of the electroviscous effect predictions by R-R&C model compared to Booth model may be attributed to the limitations invalidating the Booth model at the electrolyte conditions in this study. At high volume fractions, the R-R&C hard sphere cell model, gave a much better fit to the experimental data compared to the K-D model, which also had the advantage of being only dependent on a coefficient that linearly relates an effective volume fraction postulated for the fuzzy silica particles to the experimental. The K-D model however, depends on the intrinsic viscosity, [η], which requires the calculation of the experimental slope of the dynamic viscosity against volume fraction in the dilute limit, and also on a maximum packing fraction as a fitting parameter. Due to the effect of the fuzzy structures on the viscosity, the latter effective volume fraction ϕ_eff was calculated using two approaches: (i) as a fitting parameter by fitting the R-R&C hard sphere model to the experimental viscosity data over the entire volume fraction range, and (ii) by fitting only the linear part of the experimental viscosity at low volume fractions, It is concluded that the R-R&C hard sphere model with the effective volume fraction accounting for the fuzzy structures fits reasonably well the full range of experimental results at low and high volume fractions. When the model was used with the Adamczyk effective volume fraction (i.e., considering only the dilute region in the fitting procedure), the predictions became worse at high volume fractions where significant deviations from the experimental results were found upon the increase of pH.

Structure, thermodynamics and dynamics of the isotropic phase of spherical non-ionic surfactant micelles

We investigate a non-ionic surfactant (C(12)E(8))/water binary mixture, over a wide range of concentrations and temperatures (i.e. 1-35 wt.% of C(12)E(8) and 10-60 °C in temperature) by means of different experimental techniques: Small-Angle Neutron Scattering (SANS), Quasi Elastic Light Scattering (QELS) and High Frequency Rheology. The aims of this work are to provide information on structure, thermodynamics and dynamics of the isotropic phase of such a micellar system and, by combining these different types of information, to obtain a comprehensive image of the behaviour of this phase. Our results demonstrate that structural, thermodynamic and dynamic properties of these solutions are fully monitored by the temperature-induced changes in the ethylene-glycol chain hydration. They confirm that C(12)E(8) micelles are spherical and do not grow in the investigated range of concentrations and temperatures. They demonstrate that the interaction potential between C(12)E(8) micelles is more complicated than what was previously described, with an additional repulsive interaction. They allow us to put forward explanations for the Isotropic-Ordered phase transition as well as for the temperature behaviour of the viscosity of C(12)E(8) micellar solutions. Our investigation provides new and valuable information on the dynamics of these mixtures that reflect the complexity of the interaction potential between the C(12)E(8) micelles. It shows that concentrated solutions exhibit a viscoelastic behaviour that can be described by a simple Maxwell model.

Quantitative predictive modelling of ultrafiltration processes: colloidal science approaches

Membrane ultrafiltration is a separation process of considerable industrial importance. The specification and optimisation of such processes require the development of good predictive methods. This article reviews the development of a range of such methods based on fundamental descriptions of colloidal interactions and hydrodynamics, especially those close to the membrane surface. Particle-particle interactions may be efficiently calculated using a cell-model description of electrostatic interactions coupled with appropriate quantification of London-van der Waals forces and the entropic pressure. The overall interactions may be described in terms of osmotic pressures, which further facilitate calculation of the corresponding gradient diffusion coefficients. Methods for the calculation of the local solution viscosity are also described. Excellent agreement with experimental data is obtained when these calculated properties are incorporated in mathematical models describing either frontal ultrafiltration of cross-flow ultrafiltration. The predictive calculations may be used in a number of different ways, depending on the level of knowledge of the colloidal properties available. For example, at the most fundamental level the pressure dependence of the rate of cross-flow ultrafiltration of a protein may be calculated from a knowledge of the protein sequence.